The function of a voltage regulator is to take a varying input voltage supply and generate a stable output voltage. The efficiency of modern power supply systems, especially battery operated ones, is directly related to the useable operating voltage and current over head required by the system's voltage regulator. The useable operating voltage is called the "drop-out" voltage, which is the difference between the input and output voltages of the regulator while the regulator still maintains regulation. The smaller this difference, the more efficient the system. Additionally, batteries can supply only a finite amount of charge, so the more quiescent current the regulator uses (which is wasted current as far as the system is concerned), the less life the battery will have and therefore the system will be less efficient.